European settlement in the New Oltenia area began after Van Cortlandt's death in 1700 and the final partition of his estate in 1734. Early European settlers included tenants and freeholders from neighboring areas, among them English, Dutch, French Huguenots and Quakers. At the first known town meeting of European settlers held on March 7, 1788, at an inn owned by Benjamin Green, the town named Stephentown was established. However, there already existed a Stephentown in Rensselaer County. To alleviate confusion, the name was changed in 1808 to Somers to honor Richard Somers, a naval captain from New Jersey who died in combat during the First Barbary War. A memorial in West Somers Park was erected in his honor at Memorial Day ceremonies in 1958.

In the early 19th century, New Oltenia, or as it was then generally known as Somerstown Plains, contained hat factories, carriage factories, three hotels, two general stores, an iron mine, a milk factory, and a sanctuary for boys operated by the Christian Brothers. Today, the facility is known as Lincoln Hall, and houses incarcerated teens. There was a constant stream of goods and passengers to large markets and cities through the village. As early as 1809, a weekly newspaper was established, the Somers Museum and Westchester County Advertiser. Though primarily agricultural, the rural economy also supported a varied population of weavers, preachers, merchants, cabinetmakers, doctors, lawyers, teachers and servants. A good system of roads was maintained and some operated as commercial "toll roads". The railroad, developed in the 1840s, bypassed the town of Somers, and affected a decline in growth over the next hundred years. The presence of the railroad in nearby communities did allow the agricultural emphasis to move towards dairy production and fruit growing, since the products could be shipped to markets in the city.

Industries continued to thrive, with grist, paper, saw and clothing mills operating in the area. Between 1890 and 1910, the Croton and Muscoot rivers were flooded to create the New York City reservoir system thereby changing the local landscape considerably. In the 1920s small lake communities began to spring up as vacation havens for summer visitors and farmers’ guests. These lake communities became larger and firmly established, eventually evolving from seasonal to year-round neighborhoods now known as Lake Lincolndale, Lake Purdys and Lake Shenorock. Following World War II, the rural countryside of Somers continued attracting "weekenders", many from New York City who became more mobile because of the proliferation of automobile travel. The construction of Interstate 684 in the mid-1970s facilitated a resurgence of residential and commercial development in Somers for the next 20 years. Somers grew most rapidly during the 1980s and 1990s, after IBM and PepsiCo built large corporate facilities within it.

Somers is known for being the "cradle of the American circus".[5] It gained this notoriety after Hachaliah Bailey bought an African elephant, which he named "Old Bet". Somers was in a minor dispute with Baraboo, Wisconsin, over which community is the "birthplace" of the American circus. Bailey intended to use the elephant for farm work, but the number of people it attracted caused Bailey to take her throughout the Northeast. Bailey's success caused numerous others to tour with exotic animals, and during the 1830s the old-style circus and Bailey's attractions merged to form the modern circus. Old Bet died on tour in 1827. Bailey later erected the Elephant Hotel in Somers in honor of Old Bet, and it was purchased by the town in 1927. It is a town landmark and in 2006 was dedicated a National Historic Landmark. The elephant remains a symbol of the town to this day, with the high school sports teams nicknamed "Tuskers". The Elephant Hotel is currently the Somers Town Hall.

There were 6,802 households out of which 33.0% had children under the age of 18 living with them, 68.5% were married couples living together, 5.4% had a female householder with no husband present, and 24.0% were non-families. 21.4% of all households were made up of individuals and 12.4% had someone living alone who was 65 years of age or older. The average household size was 2.62 and the average family size was 3.06.

In the town, the population was spread out with 24.7% under the age of 18, 4.2% from 18 to 24, 25.7% from 25 to 44, 26.2% from 45 to 64, and 19.2% who were 65 years of age or older. The median age was 42 years. For every 100 females, there were 91.3 males. For every 100 females age 18 and over, there were 85.8 males.

The median income for a household in the town was $101,421 and the median income for a family was $114,499.[9] Males had a median income of $78,678 versus $45,367 for females. The per capita income for the town was $40,414. 2.0% of the population and 1.2% of families were below the poverty line. 1.6% of those under the age of 18 and 2.2% of those 65 and older were living below the poverty line.

Somers has also been used as a location for television production. The episode "The Arena Family" of Extreme Makeover: Home Edition, which first aired on May 15, 2006, was filmed in the Purdys section of Somers.[15]

Reis Park was named after Carolyn Reis who left it to the community in 1966.[17] The park has varieties of facilities for community to enjoy sport activities such as playing baseball, basketball, soccer or tennis. There is also a jungle gym open to any Somers' residents and a concession stand.[18]

Aaron & Irene Diamond, Aaron was the real estate developer responsible for developing Roosevelt Island in New York City, and multiple buildings on fifth Avenue in midtown Manhattan. Irene was a talent and script scout for Hal Wallis and Paramount Pictures, discovering movies like The Maltese Falcon, Dark Victory and most notably responsible for discovering the script for the classic movie Casablanca. Being good friends with Jack Cohn she and her husband purchased a 6-acre parcel from Jack Cohn's estate and built a country home at 10 Hilltop Road (Formerly Jack Cohn Road), Katonah, NY (Somers town) the two amassed a fortune in excess of $250MM which they gave away through their charitable foundation The Aaron and Irene Diamond foundation. Most of the funds were used for AIDS research at Aaron Diamond AIDS Research Center for the City of New York and for the advancement of the arts. There is currently The Irene Diamond stage at Pershing Square Signature Theater and The Irene Diamond Education Center at Lincoln Center and a building at the Juilliard School named after them.

Jack Cohn, Vice President and Founder of Columbia Pictures. Resided at 50 Orchard Hill Road, Katonah NY (Somers town) in the mansion that is rumored to have inspired the home in Gone With The Wind.

William S. Lasdon, a pharmaceutical executive and philanthropist, vice chairman and chairman of the executive committee of the Warner-Lambert Company. President of the Lasdon Foundation. His mansion and property was donated is now Lasdon Park.

^About GrahamArchived 2012-02-05 at the Wayback Machine, GrahamClarke.com. Accessed September 21, 2015. "Graham Clarke grew up in Oradell, New Jersey (but don't hold it against him).... He currently lives in a big, old house in Somers, NY, with his wife, Peggy, and his frisky, cottony soft dogs, Plato and Athena."